The present invention pertains generally to electronically scanned antennas and more particularly to the polyrod type of directive antenna element. An outstanding problem in naval fire control is the simultaneous tracking of multiple targets. This problem is partially solved by the use of electronically scanned antenna arrays. Due to the cost and complexity of mutual impedance and computerized steering commands, the use of these arrays has been fairly limited in the fleet.
Another configuration has a series of single radar beams produced by directive antenna elements which are serially addressed in accordance with their placement, thereby providing a steered beam. End-fire directive antenna elements have advantages over alternative types of beam directors such as parabolic reflectors, lenses, and antenna subarrays since end-fire elements occupy considerably less cross-sectional surface area. The actual length of an end-fire element however, has virtually eliminated their use.
The electromagnetic waves that can exist on a dielectric rod were first solved by Hondros and Debye and published in the Annelen der Physik, volume 32, number 8 (1910) in an article entitled Elecktromagnetische Wellen an dielektrischen Drachen, pages 465 through 476. The general theory of these modes was extended by Carson, Mead, and Schelkunoff in Hyper-frequency Waveguides--Mathematical Theory, BSTJ volume 15, page 310 (April, 1936).
U.S. Pat. No. 2,425,336, issued on the (12th of August 1947 to G. E. Mueller describes the first application of this theory in the form of a directive dielectric antenna. Following the second World War, an electronically steerable array of forty-two dielectric antennas was applied to the fire control of a U.S. Navy radar; the antenna design theory was published by Mueller and Tyrrell in a paper titled Polyrod Antennas, BSTJ volume 26, page 837 (October, 1947). The theory was based on the premise that the wave on the rod leaked as it traveled down the rod. By varying the rod diameter and the dielectric constant of the rod, the phase of leaked radiation could be adjusted in such a manner that it added constructively in the forward direction to produce a beam. Antennas could be designed that were reasonably close to practice as long as the beam widths were greater than 20 degrees. Many workers in this country and abroad have continued with this approach but failed to produce significant advances. Following the publication by Kao, Dielectric Surface Waveguides, URSI General Assembly, Ottawa, Canada, in paper 6-3.2, August 18 through 28, 1969 and the subsequent work in the field of fiber optics (dielectric rods), the basic theory became wide spread. This theory and the many confirming experiments have demonstrated that the dominant electromagnetic mode used on the antenna does not leak as it travels the rod. An alternate approach to the radiation mechanism has been developed with the electromagnetic field distribution existing around the distal end of the antenna regarded as an aperature. The extent of this field distribution determines the aperature size which in turn determines the far field radiation pattern. Zucker first recognized this approach (Theory and Application of Surface Waves, Nuovo Cimenti Suppl., volume 9, page 451, 1952); it was further expanded by Yahjian and Korhauser (A Modal Analysis of the Dielectric Rod Antenna Excited by the HE.sub.11 Mode, IEEE Transactions AP-20, number 2, page 122, March, 1972) and Zucker (Antenna Theory, Part 2, Chapter 21, McGraw-Hill, N.Y.) in the United States, Brown and Spector (The radiating Properties of End-fire Aerials, Proceedings of the IEE, 104B, page 27, 1957) in England, and E. G. Neumann (Uber das Electromagnetische Feld am Freiden Ende einer Dielektrischen Lietung I. Abstrahlung, Z. Angen Phys. 24, page 1, 1967) in West Germany
In studying the physical characteristics of end-fire dielectric rod antennas (i.e., "polyrods"), diffraction theory indicates that if D represents the maximum rod diameter and .lambda. the responsive antenna wavelength, then the minimum angle .theta. of the antenna beam within which radiation can be concentrated is proportional to .lambda./D. To achieve small angles, therefore, .lambda. must be small and D large. Both .lambda. and D are constrained however, by other system characteristics. The wavelength, .lambda., is basically restricted in radar to a limited range of wavelengths. Therefore, the only method of restricting the angle .theta. is to increase the actual length, L.sub.a, of the rod. By making L.sub.a large in a discrete elemental linear array and phasing the array for end fire (i.e., lining up a series of dipole elements and phasing each successive dipole by 90.degree. so that the beam is emitted along the line of the array), the cross-sectional dimension of the array is made independent of the actual length of the array and is restricted by only the length of a single antenna element--usually on the order of a wavelength or less which, for I band, is about three centimeters.
Dielectric rods are ideal substitutes for the directive antenna elements in a linear phased array since they are easily phased for end fire and, by their design, can be constructed of any one of a number of low loss dielectric materials available and easily matched for impedance over a wide range of frequencies.
The half power beam-width (HPBW) of dielectric rod antenna indicates that: ##EQU1## Using I band (.lambda.=3 cm), a 6.degree. HPBW requires a rod having an electrical length of approximately three meters (.about.10 ft). Even if the Hansen-Woodyard supergain relation is applied, a ten foot pole could either be used to produce a 4.degree. beam or reduced to a seven foot pole to retain a 6.degree. HPBW beam. A seven foot pole however, is still too long for use in a phased array.